Standardize on CursorShape::Underline not Underscore (#19028)

Currently terminal.cursor_shape uses `underline` and `cursor_shape` uses
`underscore`.
This standardizes them so they use the same settings value.

I think `underline` is the more common term and it matches the
terminology used by VSCode, Alacritty, iTerm, etc.

Note the protobuf enum `CursorShape::CursorUnderscore` remains
unchanged.

See also:
- https://github.com/zed-industries/zed/pull/18530
- https://github.com/zed-industries/zed/pull/17572

Release Notes:

- Settings: rename one `cursor_shape` from `underscore` to `underline`
(breaking change).

crates/language/src/buffer.rs

@@ -184,7 +184,7 @@ pub enum CursorShape {
     /// A block that surrounds the following character
     Block,
     /// An underline that runs along the following character
-    Underscore,
+    Underline,
     /// A box drawn around the following character
     Hollow,
 }

crates/language/src/proto.rs

@@ -175,7 +175,7 @@ pub fn serialize_cursor_shape(cursor_shape: &CursorShape) -> proto::CursorShape
     match cursor_shape {
         CursorShape::Bar => proto::CursorShape::CursorBar,
         CursorShape::Block => proto::CursorShape::CursorBlock,
-        CursorShape::Underscore => proto::CursorShape::CursorUnderscore,
+        CursorShape::Underline => proto::CursorShape::CursorUnderscore,
         CursorShape::Hollow => proto::CursorShape::CursorHollow,
     }
 }
@@ -185,7 +185,7 @@ pub fn deserialize_cursor_shape(cursor_shape: proto::CursorShape) -> CursorShape
     match cursor_shape {
         proto::CursorShape::CursorBar => CursorShape::Bar,
         proto::CursorShape::CursorBlock => CursorShape::Block,
-        proto::CursorShape::CursorUnderscore => CursorShape::Underscore,
+        proto::CursorShape::CursorUnderscore => CursorShape::Underline,
         proto::CursorShape::CursorHollow => CursorShape::Hollow,
     }
 }